1. Technical Field
The present invention relates generally to wireless digital communications systems and in particular to wireless digital communications systems employing code division multiple access or the like within a geographic region having empirically measurable traffic patterns. Still more particularly, the present invention relates to minimizing inter-switch handoff of expected service to mobile subscriber units within a code division multiple access wireless digital communications network.
2. Description of the Related Art
Digital telecommunications networks are increasingly based on code division multiple access (CDMA), a form of spread spectrum multiple access. Each signal within a CDMA system occupies all of the bandwidth all of the time. Separation of the signals depends on each being carried by an underlying waveform which is nearly orthogonal to all other signals. Digital codes are employed in CDMA to form the underlying quasi-orthogonal waveforms. Each user within a CDMA systems has a unique long period digital sequence called a pseudorandom sequence, which appears random to a casual observation of insufficient duration. This sequence is usually either (1) combined directly with the information stream to be sent, as in direct sequence transmission, or (2) used to select pre-planned spectrum channels among which the transmission is hopped, as in frequency hopping transmission.
CDMA provides a degree of protection against frequency-selective fading and reduces multipath interference. Signals which arrive late at the receiver do not match the portion of the code currently being utilized to decode the signal, and are thus rejected as interference. CDMA is also excellent in discriminating against noise. However, all signals within a given spectrum other than the signal of interest are treated as noise by a CDMA unit. Therefore, as more users attempt to communicate via a particular spectrum, performance for all users of that spectrum gradually degrades.
A cellular CDMA systems includes base transceiver stations (BTS's) or repeaters, referred to herein as cells, from which signals are routed into mobile switching centers (MSC), also called mobile telecommunications switching offices (MTSO) or electronic mobile exchanges (EMX) but referred to herein as simply switches. When a mobile unit moves across cell boundaries, an automatic handoff occurs. Depending on whether the two cells--current and destination--are routed to the same switch or to different switches, the type of handoff which occurs may be classified as intra-switch or inter-switch, respectively.
An intra-switch handoff is implemented as a soft handoff, in which a subscriber set near a cell boundary is served by two cells simultaneously. Concurrent service from both the old and new cells as the mobile subscriber crosses cell boundaries results in a smooth transition from cell to cell, with no noticeable change in voice quality. An inter-switch handoff, on the other hand, is implemented as a hard handoff, with the potential for a brief discontinuity of services from the two cells, where a first cell disconnects before the second cell, to which service has been handed off, completes a connection. This potential discontinuity results in a short disruption in the communication link, thereby causing a momentary degradation of the voice quality. Additionally, due to the large amount of information that needs to be transferred from one switch to another during a hard handoff, the inter-switch handoff consumes a larger amount of switch processor time than an intra-switch handoff. It is desirable, therefore, to minimize the number of inter-switch handoffs to the extent possible for typically mobile subscriber usage. The number of inter-switch handoffs may be controlled by the way cells are assigned to switches. However, cell usage by mobile subscribers and cell coverage depend in part on traffic patterns and the terrain within the cell, respectively.
Where the location and boundaries of each cell in a network and the number of switches in the network are known, it would be desirable to partition the cells among the switches in such a manner as to minimize the number of inter-switch handoffs. It would further be desirable to utilize available traffic and terrain data to partition cells in a given geographic area among the switches available to minimize the number of expected inter-switch handoffs. It would further be advantageous if the process developed could be applied to partitioning a new network based on geographical and traffic flow information, as well as to repartitioning existing networks based on operational mearurements.